Project Summary Cardiovascular disease is one of the leading causes of death in the US, and the inability to repair damaged muscle is a major obstacle in treating heart disease. Currently, there are no definitive methods to improve heart function after a heart attack, and a common result is permanent muscle loss. While a significant amount of research has focused on the ability to expand or generate cardiomyocytes for replacement of the damaged tissue, less focus has been placed on controlling the detrimental effects of fibrosis and enhancing the beneficial effects of angiogenesis. Additional information on the epicardium, or outer layer of the heart, may provide insights into these processes. Studies have demonstrated that epicardial cells differentiate into cardiomyocytes, vascular smooth muscle cells, endothelial cells, and fibroblasts, and an in depth knowledge of the mechanisms that cause these cell populations to form in the embryonic heart will be essential for programming the adult epicardium to generate these cell types after heart injury. Platelet derived growth factor (PDGF) signaling pathways are essential for normal epicardial development, and the main goal of this proposal is to discover how PDGFs direct epicardial cell development and differentiation. This goal will be accomplished using epicardial-specific loss-of-function alleles of the PDGF receptors in the mouse. The specific aims of this proposal are: 1) To determine the mechanism of PDGF receptor action in the initial development of the epicardium; 2) To elucidate the requirement for PDGFR in coronary vascular smooth muscle cells and cardiac fibroblasts; and 3) To determine if PDGFR signaling is required for fibroblast development and function within the heart. Understanding the mechanisms of PDGF receptor function in the epicardium and epicardial derived cells will provide important insights into the signaling mechanisms governing epicardial cell biology and potentially reveal signaling pathways that could be manipulated to control fibrosis and direct angiogenesis after cardiac injury.